I. Field of the Invention
This invention relates generally to a method and apparatus for assessing the efficiency of a patient""s body. More particularly, the present invention relates to a method and apparatus for estimating the efficiency of the body of a patient suffering from chronic congestive heart failure (CHF) while undergoing different cardiac therapies, in order to determine the best therapy for the patient. The efficiency of a patient""s body while undergoing a therapy can be determined by measuring and calculating the patient""s accelerationxe2x80x94heart rate ratio while undergoing the therapy. More than one type of therapy can be performed on a patient and the accelerationxe2x80x94heart rate ratios, or efficiency indices, of the therapies can be compared to determine the best cardiac therapy for the patient.
II. Description of the Related Art
The heart muscle derives its energy almost entirely from aerobic metabolism, which means it consumes oxygen. A healthy heart requires less oxygen, or uses less energy, than an unhealthy heart does to achieve a given cardiac output.
Heart rate is an important determinant of myocardial oxygen and energy consumption. The energy cost per minute of the heart must equal the energy cost per beat times the number of beats per minute. Effectively, a more efficient heart has a lower corresponding heart rate for any given stroke volume.
For CHF patients especially, it is important to maintain a low heart rate, thereby reducing the energy demand of the heart. Heart rates of CHF patients are artificially kept low by using drugs such as beta-blockers. However, CHF patients typically manifest abnormally low cardiac output as a result of their condition. In order to still get an adequate amount of blood flow to the body of these patients, pacing therapy has been used. Implantable cardiac rhythm management devices have been used to enhance hemodynamic performance of the heart by pacing the left ventricle and appropriately setting the atrial-ventricular delay (AVD) parameter. However, it is unclear the effect that pacing therapy has on the heart rate of CHF patients.
Attempts have been made to develop clinically practical methods for estimating energy expenditure during various activities. Several studies have used accelerometers, which measure body movement, to estimate total body energy expenditure of the patient during specific laboratory exercise tests or daily life activities. Some articles detailing such studies include: xe2x80x9cEvaluation of Commercial Accelerometer (Tritrac-R3 D) to Measure Energy Expenditure During Ambulationxe2x80x9d in the International Journal of Sports Medicine (Vol. 19, January, 1998, pp. 43-47); xe2x80x9cAbility of the Caltrac Accelerometer to Assess Daily Physical Activity Levelsxe2x80x9d in the Journal of Cardiopulmonary Rehabilitation (Vol. 15, March/April, 1995, pp. 107-113); and xe2x80x9cImproving Energy Expenditure Estimation by Using a Triaxial Accelerometerxe2x80x9d in the Journal of Applied Physiology (Vol. 83, No. 6, December, 1997, pp. 2112-2122). One such study, in particular, as detailed in xe2x80x9cAssessment of Energy Expenditure for Physical Activity Using a Triaxial Accelerometerxe2x80x9d in Medicine and Science in Sports and Exercise (Vol. 26, No. 12, December 1994, pp. 1516-1523), found a high correlation (r=0.96) between energy expenditure and acceleration in the antero-posterior direction during walking. The study indicated that it was possible to obtain an estimate of energy expenditure from acceleration measurements.
A method is still needed that will allow physicians to determine whether a patient""s body is efficient or not during activity. A high level of activity with a corresponding low heart rate indicates efficiency. In particular, a method is needed that will allow a physician to determine the efficiency of a patient""s body while the patient undergoes different cardiac therapies, and thereby determine the best therapy for the patient. The inventive method and apparatus allow for identification of the best cardiac therapy for a patient.
After considering the related art, it can be seen that a need exists for a method and apparatus to determine the efficiency of a patient""s body while undergoing certain cardiac therapies, and, in particular, the efficiency of a CHF patient""s body. The present invention meets the need for such a method and apparatus. It uses an accelerometer to measure the activity of a patient, which, as the prior art indicates, establishes the level of energy that is expended by a patient during the activity. The invention also measures the heart rate of a patient over time. The ratio of acceleration and heart rate is then calculated for each cardiac therapy performed on the patient. The accelerationxe2x80x94heart rate ratios for the tested therapies are then compared to determine which cardiac therapy is the best for the patient. The best therapy will be the one that allows the most acceleration, or activity, with the lowest corresponding heart rate. The best therapy will have the highest acceleration heart rate ratio, otherwise called the xe2x80x9cefficiency index.xe2x80x9d Overall, the described method and apparatus meet the existing need for the invention.
In its simplest embodiment, the invention comprises measuring the values of acceleration and heart rate for a patient, and calculating the ratio of acceleration to heart rate. The ratio is the patient""s efficiency index. The invention may also be used to evaluate the effect of different cardiac therapies on the patient, by measuring acceleration and heart rate values during cardiac therapies and by calculating the accelerationxe2x80x94heart rate ratio that correspond to the therapies. The ratios may be compared, and the therapy with the highest accelerationxe2x80x94heart rate ratio, or efficiency index, is determined to be the best therapy for the patient. The best therapy is the one that allows the patient to have the most acceleration, or activity, with the lowest corresponding heart rate.